Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
There are many different solar cell architectures. Two common designs are the selective emitter (SE) and the interdigitated backside contact (IBC). A SE solar cell has high-dose stripes across the lightly doped surface impinged by sunlight. An IBC solar cell has alternating p-type and n-type stripes across the surface not impinged by sunlight. Both a SE and IBC solar cell may be implanted with ions to dope the various regions.
“Glitches” may occur during the ion implantation process. A glitch is defined as a sudden degradation in the beam quality during an ion implantation operation, typically due to a variation in an operating voltage, which potentially renders the workpiece unusable. Such a glitch is typically caused by interactions between components along the beam path, which affect one or more operating voltages, and can be caused at various locations along the beam path. For example, ion implanters generally employ several electrodes along this beam path, which accelerate the beam, decelerate the beam, or suppress spurious streams of electrons that are generated during operation. Each of these electrodes is maintained at a predetermined voltage. Often, electrodes of different voltage are located near each other and therefore an arc may occur between electrodes. Generally, arcs occur across acceleration gaps, deceleration gaps, or suppression gaps, although arcs may occur elsewhere. Interaction between, for example, a source extraction voltage, source suppression voltage, and source beam current may cause a glitch. These glitches may be detected as a sharp change in the current from one of the power supplies. If the implantation is interrupted or affected by a glitch, the implanted solar cell or other workpiece may be negatively affected. For example, a solar cell may have a lower efficiency due to the lower implanted dose caused by a glitch. This may have a cost impact on the implanted workpieces. Thus, steps are usually taken to both minimize the occurrence of such glitches and to recover from the glitches if possible.
FIG. 1 is a chart illustrating a glitch. The beam current is set to a predetermined value 10. The glitch 11 occurs during the period marked Δt outlined by the dotted lines 12, 13 where the beam current drops below the predetermined value 10. Minimizing the Δt period means that there is less negative impact on the workpiece being implanted. The glitch 11 may be sensed by measuring changes in voltage or current. An arc is typically sensed by either an abrupt voltage collapse, or an abrupt current surge. When a glitch is detected, one solution is to immediately reduce the ion beam current to zero, thus terminating the implantation at a defined location on the workpiece. This is referred to as “blanking the beam”. FIG. 2 is a chart illustrating blanking an ion beam. At time 100 when the glitch is first detected, the voltage is dropped to zero and then slowly built back up to the desired voltage level. At this time, implantation stops as well, and the position at this time is saved. In one instance, the voltage may be blanked for tens of milliseconds before voltage is recovered over the next hundred or more milliseconds. When the voltage recovers within 0.1-0.5% of the desired value, such as at time 101, implantation may continue from the location where it had stopped. Thus, once the glitch condition has been remedied, the implantation process ideally resumes at exactly the same location on the workpiece, with ideally the same beam characteristics as existed when the glitch was detected. The goal is to achieve a uniform doping profile, and this can be achieved by controlling the beam current or the workpiece scan speed (exposure time). However, blanking is time-consuming, which has a negative impact on throughput. Decreased throughput also results in higher costs.
Repairing the dose loss caused by the glitch in such a manner may take over 30 seconds, which may be too time-consuming for the throughput demands of the solar cell industry. Ion beam stability and implant uniformity within the ion implanter are controlled by the speed of the voltage and current sources connected to the ion implanter.
Therefore, there is a need in the art for an improved method of glitch recovery during the implantation of workpieces and, more particularly, solar cells.